just use incendiary devices. A somewhat decent player wont let u kill it with pio flamer (251 flamer sets a house on fire very fast but its hard too), but if you use LeIg incendiary rounds (Feuersturm doc) or Mortar HT incendiary barrages you can counterpick a FHQ spammer with ease
Come on aerafield i play with and against you a lot and we both know that a doctrinal flame mortar ht for
1. is doctrinal because no stock units are up to the task until very late game.
2. the flame mortar halftrack is still ineffective against a lot of the larger buildings and take ages to set alite not to mention a costly munitions bill.